The present invention relates to an anti-skid control system for use in a motor vehicle, and more particularly to such an anti-skid control system for continuously controlling the grade (gradient) of the braking pressure for a wheel of the motor vehicle in accordance with variation of a slipping state of the wheel.
A conventional anti-skid control system is arranged such that a wheel speed and a wheel acceleration are respectively compared with a plurality of reference speeds and a plurality of reference accelerations so as to control the braking pressure for a wheel on the basis of the comparison results in accordance with predetermined pressure-increasing patterns or pressure-decreasing patterns. However, in the case of controlling the braking pressure in accordance with the predetermined fixed pressure-controlling pattern, difficulty is encountered to effect a braking control operation adequately coping with the slipping state of the wheel, thereby deteriorating the braking efficiency.
One possible solution is, as disclosed in the Japanese Patent Provisional Publication No. 1-182155, to continuously change the pressure-increasing or pressure-decreasing grade through the duty control of the wheel braking pressure in which the duty ratio is determined in accordance with the slip ratio of the wheel and the time variation of the slip ratio. According to this conventional anti-skid control system, since in addition to the wheel slip ratio the time variation of the slip ratio is also used as a braking pressure control parameter, when the wheel slip ratio varies, the duty ratio can be changed in accordance with the slip ratio variation, thus preventing the wheel slip ratio from being greatly deviated from a target value. Here, for example, in the case of increasing the wheel braking pressure, the conventional anti-skid control system is arranged to alternately effect the pressure-increasing and pressure-maintaining operations in accordance with the duty ratio determined by the wheel slip ratio and the time variation of the slip ratio. When the wheel braking pressure is switched from the maintaining state to the increasing state, vibration of the wheel and the like occur due to the variation of the braking pressure. The occurrence of the wheel vibration causes variation of the wheel speed whereby errors appear in the slip ratio and the slip ratio time variation to be obtained on the basis of the wheel speed. If determining the duty ratio on the basis of the slip ratio and slip ratio time variation includes the errors, difficulty is encountered to adequately keep the slipping state of the wheel.